260 



GENETICS IN RELATION TO AGRICULTURE 



biotypes can be differentiated from one another so that they "breed 

 true." How these distinct biotypes originate will be considered in the 

 following chapter, the fact that they exist is the chief consideration here. 

 The effect of "mass" selection in causing temporary changes in heteroge- 

 neous varieties of plants and races of animals is easily understood by 

 the aid of the diagram shown in Fig. 108. The area within the large 

 curve represents a mixed population or phenotypically similar group 

 containing a number of distinct genotypes indicated by the small curves 

 A-Z. Every genotype has its own variation curve and is distinct from 

 each of the others, but they intergrade with each other so completely 

 that the population appears as an entity. Now if one should select 

 individuals from either extreme of the population, say at 90 or 70, it is 

 clear that such individuals might belong to any one of four or five geno- 





Fio. 108. Schematic diagiam showing the relation of a population to the biotypes 

 composing it, or of a phenotype, to the genotypes or pure lines within it. (After Lang 

 from Goldschmidt.) 



types. If selection in the same direction were continued a strain would 

 be established with a mode distinct from the mode of the original popu- 

 lation. These strains could be maintained by continual selection 

 and in time a single genotype might be isolated when selection would 

 be said to have changed the type permanently. But selection changed 

 nothing it only isolated a certain genotype or genotypes from the origi- 

 nal mixture. Tower's results in selecting for the purpose of creating 

 albinic and melanic strains of beetles as illustrated in Fig. 109 may be 

 explained in this way. The original population shown at A consisted 

 of a number of distinct biotypes. By the isolation of several extreme 

 variants Tower separated plus and minus strains which he was able 

 to maintain for eight generations by practising intensive selection. In 

 the eighth generation he divided each population in half, continuing in- 

 tensive selection with one portion and stopping all selection in the other. 

 By this method he was able to maintain the plus and minus strains and at 

 the same time to observe that in the ninth generation the mode of the 



